Prepend ## to every line in a text file

Question

I want to add hashes to all the lines in a regular text file. I'm fine with both the use of terminal and GUI—I just need to get it done.

score 57 · Accepted Answer · edited Jul 31 '11 at 11:28

57

You can use sed to do that:

sed -i.bak 's/^/##/' file

This replaces the start of the line (^) with ##.

With the -i.bak switch, sed edits the file in-place, but creates a backup copy with extension.bak.

edited Jul 31 '11 at 11:28

dv3500ea

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answered Jul 30 '11 at 06:48

Jukka Matilainen

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+1 This is the first time I encounter this backup method. I always did an inline replace directly: `sed 's/^/##/' -i file`. In this case, this would be preferred since it cannot go wrong. In other cases, this command can be combined with `diff -u file.bak file` to see the differences (if necessary, pipe it through `less`). If it's confirmed to work, the backup can be removed. Otherwise it could be restored with a simple `mv file.bak file`. – Lekensteyn Jul 30 '11 at 08:28
1

Elegant. I was thinking about something like `sed 's/^$.*$$/##\1/'`, but this is much nicer. – arrange Jul 31 '11 at 08:54
Nice one.. I was thinking about suggesting Vim to do this... – Sathish Aug 06 '11 at 13:28
This helped me convert a block list file :) Thank you 1+ – Neil Nov 22 '15 at 04:25

score 8 · Answer 2 · answered Jul 31 '11 at 03:43

8

Here is a solution to this problem using perl

perl -e 'while (<>) {print "##$_"}' < infile > outfile

answered Jul 31 '11 at 03:43

beav_35

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The `-p` switch is also useful: `perl -pe 's/^/##/' infile > outfile`. (There's also the `-i[extension]` switch for replacing the target file in-place.) http://perldoc.perl.org/perlrun.html#%2a-p%2a – Jukka Matilainen Jul 31 '11 at 07:51

score 7 · Answer 3 · edited May 23 '17 at 12:39

7

While we are at it:

gawk -i inplace '{print "##"$0}' infile

This uses the (comparatively new) inplace editing plugin for GNU awk 4.1.0+.

edited May 23 '17 at 12:39

Community

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answered Sep 17 '14 at 23:49

muru

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Eliah Kagan · Answer 4 · 2017-11-09T06:24:21.060

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Here's a bash way:

while read -r; do printf '##%s\n' "$REPLY"; done < infile > outfile

(In the bash shell, running read -r with no other arguments works like IFS= read -r REPLY.)

This is stylistically inspired by beav_35's perl solution, which I admit probably runs much faster for huge files, since perl may be expected to be more efficient than a shell when it comes to text processing.

edited Nov 09 '17 at 06:24

answered Sep 17 '14 at 23:42

Eliah Kagan

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score 4 · Answer 5 · answered Sep 17 '14 at 23:45

4

sed -i is not POSIX-standard, so if you are a purist you will want to use ed:

printf ",s/^/##/\nw\nq" | ed -s file.txt

answered Sep 17 '14 at 23:45

fkraiem

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score 3 · Answer 6 · answered Jun 12 '15 at 17:35

3

Here's an easier perl way than presented elsewhere:

perl -pi -e 'print "##"' YOURFILEHERE

This (ab)uses the fact that perl -p prints the line after executing the command given in -e.

answered Jun 12 '15 at 17:35

abligh

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score 0 · Answer 7 · answered Apr 16 '16 at 18:38

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You can use Vim in Ex mode:

ex -sc '%s/^/##/|x' file

% select all lines
s substitute
x save and close

answered Apr 16 '16 at 18:38

Zombo

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score 0 · Answer 8 · answered Dec 24 '16 at 23:32

Can be done with python's mapping function and redirecting stdin:

$ cat input.txt                                                                                                          
lorem ipsum
quick brown fox
hello world
$ python -c 'import sys;print "".join(map(lambda x: "##"+x,sys.stdin.readlines()))'  < input.txt                         
##lorem ipsum
##quick brown fox
##hello world

Save the output to new file and use it instead of original

Prepend ## to every line in a text file

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